It is well-established that crop biomass accumulation is a linear function of cumulative crop transpiration (de Wit, 1958; Tanner and Sinclair, 1983; Musick et al., 1994), and that the linear coefficient relating SY to T is WUE. Some have argued that HI is independent of T (Spaeth et al., 1984), but HI does decline when water stress is extremely severe (Specht et al., 1986).
Crop WUE differs substantially over locations and years (de Wit, 1958), primarily because evaporative demand is driven by the leaf-to-air vapor pressure deficit (VPD).
Agronomic practices greatly influence T, but have little impact on WUE. For example, when Jones (1992) plotted yields produced by various agronomic treatments (e.g., planting dates, soil fertility levels, etc.) at a given test site against the seasonal crop water use of each treatment, all of the treatment data points fell on the linear yield-to-water regression trend line, indicating a common WUE.
The objective of this study is to identify the effects of permanent raised beds on crop performance and WUE in arid areas of China, and we also present suggestions for further research to enhance the development of permanent raised bed systems in China.
Wheat yield was also greater in PRB than in TT and ZT treatments, so the mean WUE for PRB treatments was 11-29% greater than TT and ZT for all 3 years of the experiment (P< 0.05).
Field experimental results reported here demonstrate that PRB production was associated with a substantial and significant improvement in WUE in all 3 years compared with TT or ZT practice, and an increase (significant in 1 year only) in the mean yield of wheat compared with TT.
Water use efficiency (WUE) of spring growth ryegrass grown on Atkins and Pope soils over two years N Rate kg [ha.sup.-1] Soil Soil series Year 0 42 84 126(*) mean kg [ha.sup.-1] cm [H20.sup.-1] Atkins 1989 78.6(***) 125 145 195 1990 28.1 69.0 89.1 131 Mean 53.4 97.1 117 163 108b(**) Pope 1989 132 166 200 202 1990 61.3 118 145 131 Mean 96.8d 142c 173a 166b 144a N rate mean 75.1d 120c 145b 165a * N rate reflects split application.
In contrast, the WUE (71.0 kg [ha.sup.-1] cm [H.sub.2]0) of summer-fall growth ryegrass at the 84 kg [ha.sup.-1] N fertilization rate on the well drained Pope soil (Table 2) was similar to those previously reported for summer-fall growth orchardgrass and less than for tall fescue fertilized at similar N rates on well drained upland soils of similar depth (Sour et al., 1991; Stout 1992).
There was a significant N fertilization main effect and a significant N fertilization by soil interaction in WUE of spring growth ryegrass (Table 1).
Although good correlations were found between both yield and [ET.sub.a] with [[Psi].sub.T] (Dean et al., 1996), WUE of bermudagrass exhibited no significant changes in response to changes in water stress imposed (Table 2).
The WUE remained fairly constant in bermudagrass with decreasing [[Psi].sub.T].
Abbreviations: EC, electrical conductivity; [EC.sub.e], salinity; [ET.sub.a], actual evapotranspiration; [Et.sub.o], daily potential evapotranspiraton; LF, leaching fraction; WUE, water use efficiency; [[Psi].sub.L], leaf xylem water potential; [[Psi].sub.M], soil matric potential; [[Psi].sub.T], total soil water potential; [[Psi].sub.II], osmotic potential; [[Psi].sub.II--TISS], tissue osmolality.(*), (**), (***) Significant at the 0.05, 0.01, and 0.001 probability levels, respectively.